Diffusion Monte Carlo study of the equation of state of solid ortho-D2_2

We present results of Diffusion Monte Carlo calculations for a system of solid ortho-D_2 at different densities, for pressure ranging from 0 up to 350MPa. We compare the equation of state obtained using two of the most used effective intermolecular potentials, i.e. the Silvera--Goldman and the Buck potentials, with experimental data, in order to assess the validity of the model interactions. The Silvera-Goldman potential has been found to provide a satisfactory agreement with experimental results, showing that, as opposed to what recently found for p-H_2, three--body forces can be efficiently accounted for by an effective two--body term.Comment: 11 pages, 4 figure

Comet C/2004 Q2 (MACHHOLZ): Parent Volatiles, a Search for Deuterated Methane, and Constraint on the CH4 Spin Temperature

High-dispersion (l/dl ~ 25,000) infrared spectra of Comet C/2004 Q2 (Machholz) were acquired on Nov. 28-29, 2004, and Jan. 19, 2005 (UT dates) with NIRSPEC at the Keck-2 telescope on Mauna Kea. We detected H2O, CH4, C2H2, C2H6, CO, H2CO, CH3OH, HCN, and NH3 and we conducted a sensitive search for CH3D. We report rotational temperatures, production rates, and mixing ratios (with respect to H2O) at heliocentric distances of 1.49 AU (Nov. 2004) and 1.21 AU (Jan. 2005). We highlight three principal results: (1) The mixing ratios of parent volatiles measured at 1.49 AU and 1.21 AU agree within confidence limits, consistent with homogeneous composition in the mean volatile release from the nucleus of C/2004 Q2. Notably, the relative abundance of C2H6/C2H2 is substantially higher than those measured in other comets, while the mixing ratios C2H6/H2O, CH3OH/H2O, and HCN/H2O are similar to those observed in comets, referred to as "organics-normal". (2) The spin temperature of CH4 is > 35-38 K, an estimate consistent with the more robust spin temperature found for H2O. (3) We obtained a 3s upper limit of CH3D/CH4 < 0.020 (D/H < 0.005). This limit suggests that methane released from the nucleus of C/2004 Q2 is not dominated by a component formed in extremely cold (near 10 K) environments. Formation pathways of both interstellar and nebular origin consistent with the measured D/H in methane are discussed. Evaluating the relative contributions of these pathways requires further modeling of chemistry including both gas-phase and gas-grain processes in the natal interstellar cloud and in the protoplanetary disk.Comment: Accepted by The Astrophysical Journa

Self-Motions of General 3-RPR Planar Parallel Robots

This paper studies the kinematic geometry of general 3-RPR planar parallel robots with actuated base joints. These robots, while largely overlooked, have simple direct kinematics and large singularity-free workspace. Furthermore, their kinematic geometry is the same as that of a newly developed parallel robot with SCARA-type motions. Starting from the direct and inverse kinematic model, the expressions for the singularity loci of 3-RPR planar parallel robots are determined. Then, the global behaviour at all singularities is geometrically described by studying the degeneracy of the direct kinematic model. Special cases of self-motions are then examined and the degree of freedom gained in such special configurations is kinematically interpreted. Finally, a practical example is discussed and experimental validations performed on an actual robot prototype are presented

Hydrogen-Helium Mixtures in the Interiors of Giant Planets

Equilibrium properties of hydrogen-helium mixtures under conditions similar to the interior of giant gas planets are studied by means of first principle density functional molecular dynamics simulations. We investigate the molecular and atomic fluid phase of hydrogen with and without the presence of helium for densities between $\rho=0.19$ g$$cm$^{-3}$ and $\rho=0.66$ g$$cm$^{-3}$ and temperatures from $T=500$K to $T=8000 {K}$. Helium has a crucial influence on the ionic and electronic structure of the liquid. Hydrogen molecule bonds are shortened as well as strengthened which leads to more stable hydrogen molecules compared to pure hydrogen for the same thermodynamic conditions. The {\it ab initio} treatment of the mixture enables us to investigate the validity of the widely used linear mixing approximation. We find deviations of up to 8% in energy and volume from linear mixing at constant pressure in the region of molecular dissociation.Comment: 13 pages, 18 figures, submitted to PR

Diagrammatic quantum field formalism for localized electrons

We introduce a diagrammatic quantum field formalism for the evaluation of normalized expectation values of operators, and suitable for systems with localized electrons. It is used to develop a convergent series expansion for the energy in powers of overlap integrals of single-particle orbitals. This method gives intuitive and practical rules for writing down the expansion to arbitrary order of overlap, and can be applied to any spin configuration and to any dimension. Its applicability for systems with well localized electrons has been illustrated with examples, including the two-dimensional Wigner crystal and spin-singlets in the low-density electron gas.Comment: 13 pages, 0 figure

Phase separation in hydrogen-helium mixtures at Mbar pressures

The properties of hydrogen-helium mixtures at Mbar pressures and intermediate temperatures (4000 to 10000 K) are calculated with first-principles molecular dynamics simulations. We determine the equation of state as a function of density, temperature, and composition and, using thermodynamic integration, we estimate the Gibbs free energy of mixing, thereby determining the temperature, at a given pressure, when helium becomes insoluble in dense metallic hydrogen. These results are directly relevant to models of the interior structure and evolution of Jovian planets. We find that the temperatures for the demixing of helium and hydrogen are sufficiently high to cross the planetary adiabat of Saturn at pressures around 5 Mbar; helium is partially miscible throughout a significant portion of the interior of Saturn, and to a lesser extent in Jupiter.Comment: 6 pages, 7 figures. Published in "Proceedings of the National Academy of Sciences USA

A TWO-DIMENSIONAL MATHEMATICAL MODEL OF THE HUMAN BLOOD GLUCOSE REGULATION MECHANISM

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The ϵ\epsilon-ζ\zeta Transition in Solid Oxygen

The structure of solid oxygen has been studied at pressures from 50 to 140~GPa using static structure search methods and molecular dynamics simulations with density functional theory and a hybrid exchange functional. Several crystalline structures with space group symmetries {\it Pnma}, {\it P}\,2$_{1}${\it /m}, {\it Pm} and {\it P}\,6$_3$/{\it mmc} have been identified as candidates for the $\zeta$ phase of oxygen at 0~K. Within the hybrid exchange functional framework and at 300~K temperature, {\it Pm} is shown to be energetically most favorable above 111~GPa. A comparison with experimental X-ray diffraction, spectroscopic and superconductivity measurements is provided for all competing structures.Comment: 6 pages, 5 figure

A comprehensive study of infrared OH prompt emission in two comets. I. Observations and effective g-factors

We present high-dispersion infrared spectra of hydroxyl (OH) in comets C/2000 WM1 (LINEAR) and C/2004 Q2 (Machholz), acquired with the Near Infrared Echelle Spectrograph at the Keck Observatory atop Mauna Kea, Hawaii. Most of these rovibrational transitions result from photodissociative excitation of H_2O giving rise to OH "prompt" emission. We present calibrated emission efficiencies (equivalent g-factors, measured in OH photons s^(-1) [H_2O molecule]^(-1)) for more than 20 OH lines sampled in these two comets. The OH transitions analyzed cover a broad range of rotational excitation. This infrared database for OH can be used in two principal ways: (1) as an indirect tool for obtaining water production in comets simultaneously with the production of other parent volatiles, even when direct detections of H_2O are not available; and (2) as an observational constraint to models predicting the rotational distribution of rovibrationally excited OH produced by water photolysis